Transformers primary in serie

Thread Starter

wimv

Joined Nov 26, 2017
4
Hi all,
I've been looking around on the net for a while and posted my problem on some fora but I still don't have an answer I can rely on. (answers going from "this will work fine" to "this will give sparkles")
The situation: I have a vintage amplifier (Sansui AU-717), US-model =120V. But I'm staying in EU = 230V.
It has a massive power supply with 2 separate transformers, 1 for each channel. For the moment I am using a 1000VA stepdown transformer and that works fine but it's quite a block.
Lately I read about connecting transformers primary in serie.
So I'm asking, is it possible to serial connect the primary of those two identical transformers and charge them with 230V instead of the 120V, leaving the secondaries as they are, and obtain a reasonably equal output.
Is this wishfull thinking or can it be a practical solution.
Primary serie.png
Thanks very much for clearing things up for me!
Wim
 

ericgibbs

Joined Jan 29, 2010
9,346
hi wimv,
Welcome to AAC.
Have you considered connecting the two 115v primaries in series, for 230Vac operation.
Then connect 3 of the 39Vac in series to give 117Vac.?
E
 

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Thread Starter

wimv

Joined Nov 26, 2017
4
hi wimv,
Welcome to AAC.
Have you considered connecting the two 115v primaries in series, for 230Vac operation.
Then connect 3 of the 39Vac in series to give 117Vac.?
E
Thanks for welcoming me and all the reactions.

It surprises me that nearly all reactions talk about linking the secondaries (parallel). Does it mean it is required to connect the secondary when you do so for the primary? That's not what I want. I want to keep the secondaries separated like the original situation and only put the primary in serie so I can power the device with 230V instead of 120V and keep the 39V output.
Does it make a difference when connecting the primaries in serie when they are on the same core or when they are on two different cores like in this case?
Wimv
 

ericgibbs

Joined Jan 29, 2010
9,346
hi wimv,
I assumed that you were not planning to use any of the 39V secondary's and that you wanted to use two identical 115V primary transformers to allow operation of a 115Vac powered unit from the Euro mains of 230Vac.???
E
 

Thread Starter

wimv

Joined Nov 26, 2017
4
Hi Eric,
It's about the power supply of an amplifier, so I absolutely want to preserve the 39V out, I don't want to touch the secondaries. So inded, I only want to make a change by putting the primaries in serie so I can power the device with 230V instead of the 120V (where the device was build for) and still have approximately 39V out of the two secondaries.
In simple theory this seems ok but I like to have some confirmation from a specialist that this works.
The two transformers are identical and I guess in the amplifier the load on each will be equal also.
Wimv
 

ericgibbs

Joined Jan 29, 2010
9,346
hi wimv,
Providing the two transformer secondary current loads are balanced it should work OK.
To try to get an answer I have run some LTSpice sims under in balance and off balance loads.
The voltage across the two primaries will not be shared equally ie: at 115Vac if the secondary loads are not equal.
The off balance variation in voltage across the two primaries are IMO within specification, providing the secondary loads are operating within 25%.

If it was my project I would test try and closely observe for any side effects, heating etc.

E
 

Thread Starter

wimv

Joined Nov 26, 2017
4
Ok, thanks very much, one of those weeks I'll detach the transformers and give it a secure try.
Will update on this later.
Wim
 

crutschow

Joined Mar 14, 2008
24,099
It surprises me that nearly all reactions talk about linking the secondaries (parallel). Does it mean it is required to connect the secondary when you do so for the primary?
What is that a surprise?
That's the only way it will work.

Unless the secondary loads are exactly matched (and that is not likely with the two stereo channels) then putting the primaries in series without changing the secondaries will give bad results, and could damage the amplifier with one channel experiencing an overvoltage.
 
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